Process for the preparation of 3,3,3-trifluopropene

ABSTRACT

The present invention provides a process for preparing 3,3,3-trifluoropropene (1243zf), the process comprising: (a)fluorinating CCI 3 CH 2 CH 2 CI (250fb) to produce a reaction product comprising CF 3 CH 2 CH 2 CI (253fb) in the liquid phase in a first reactor, using HF as the fluorinating agent; and (b)(i) dehydrohalogenating 253fb to produce 1243zf in the vapour phase in the present of a catalyst in a second reactor; or (b)(ii) dehydrohalogenating 253fb to produce 1243zf in a second reactor, wherein the reaction product comprising 253fb produced in step (a) has subjected to one or more purification steps before step (b). The present invention also provides an azeotropic or near-azeotropic composition comprising HF and 253fb.

The invention relates to a process for preparing 3,3,3-trifluoropropene.

3,3,3-trifluoropropene, which is also known as HFO-1243zf (or 1243zf),is a useful monomer for the production of fluorosilicones, and in themanufacture of trifluoropropene epoxide and3,3,3-trifluoropropylbenzene. 1243zf is also believed to have utility inrefrigerant compositions.

The listing or discussion of a prior-published document in thisspecification should not necessarily be taken as an acknowledgement thatthe document is part of the state of the art or is common generalknowledge.

U.S. Pat. No. 5,986,151 describes the preparation of 1243zf startingfrom CF₃CH₂CF₂H, involving a complicated series of separatedehydrofluorination and hydrogenation reactions.

U.S. Pat. No. 4,220,608 describes the preparation of 1243zf by reactingat least one of 1,1,1,3-tetrachloropropane (also known as 250fb),1,1,3-trichloroprop-1-ene and 3,3,3-trichloropropene with hydrogenfluoride (HF) in the presence of a nitrogen-based catalyst. Suchcatalysts are not ideal, for example because they cannot easily beregenerated or separated from the reagents and/or products.

U.S. Pat. Nos . 2,889,379 and 4,465,786 both describe the preparation of1243zf by the reaction of a halogenated hydrocarbon (e.g. 250fb) with HFin the presence of (modified) chromium oxyfluoride catalysts. Theactivity, selectivity, robustness and/or ease of regeneration of suchcatalysts are not ideal.

The subject invention addresses the above and other deficiencies in theart by the provision of a process for preparing 3,3,3-trifluoropropene(1243zf), the process comprising:

-   -   (a) fluorinating CCl₃CH₂CH₂Cl (250fb) to produce CF₃CH₂CH₂Cl        (253fb) in the liquid phase in a first reactor; and    -   (b) dehydrohalogenating 253fb to produce 1243zf in the vapour        phase in the presence of a catalyst in a second reactor.

The present invention also provides a process for preparing3,3,3-trifluoropropene (1243zf), the process comprising:

-   -   (a) fluorinating CCl₃CH₂CH₂Cl (250fb) to produce a reaction        product comprising CF₃CH₂CH₂Cl (253fb) in the liquid phase in a        first reactor, using HF as the fluorinating agent; and    -   (b) dehydrohalogenating 253fb to produce 1243zf in a second        reactor;        -   wherein the reaction product comprising 253fb produced in            step (a) is subjected to one or more purification steps            before step (b).

250fb may be purchased from common suppliers of halogenatedhydrocarbons, such as Apollo Scientific, Stockport, UK. Alternatively,250fb may be prepared by the telornerisation of carbon tetrachloride(CCl₄) and ethylene (see, for example, J. Am. Chem. Soc. Vol. 70, p2529, 1948, which is incorporated herein by reference).

The conversion of 250fb to 1243zf typically involves fluorination anddehydrohalogenation steps.

For example, 250fb may be fluorinated to produce CF₃CH₂CH₂Cl, asillustrated in the scheme below. 1243zf may be produced by a finaldehydrochlorination step of CF₃CH₂CH₂Cl. This is illustrated below:

Step (a) of the process of the invention is typically conducted at atemperature of from about 20 to about 100° C., preferably from about 40to about 70° C.

Step (a) is typically conducted at a pressure of from about 100 to about1000 kPa (about 1 to about 10 barg), preferably from about 200 kPa toabout 700 kPa (about 2 to about 7 barg).

In one aspect, step (a) may be conducted at a temperature of from about40 to about 70° C. and a pressure of from about 200 kPa to about 700kPa.

Step (a) may be conducted in the presence of a polymerisation inhibitorand/or retarder. Any suitable polymerisation inhibitor/retarder may beused. Suitable polymerisation inhibitors include, but are not limitedto, cyclic ketone or quinone-based aromatic compounds, nitro- ornitrogen-containing compounds, or sulphur-containing compounds, e.g.cyclobutanone and cyclohexanone and mixtures thereof. Without wishing tobe bound by theory, it is believed that the use of a polymerisationinhibitor/retarder minimises tar formation.

Step (a) is typically conducted in the presence of a catalyst. Suitablecatalysts include Lewis acid catalysts. Suitable Lewis acid catalystsinclude, but are not limited to, TiCl₄, BF₃, SnF_(x)Cl_(y) (whereinx+y=4) such as SnCl₄ or SnCl₂F₂, TaF₅, SbCl₅ and AlCl₃. A preferredcatalyst for use in step (a) is SnF_(x)Cl_(y).

SnCl₄ is readily available. SnCl₄ will be fluorinated in the presence ofHF to form mixed Cl/F species and/or SnF₄. Therefore, in one aspect ofthe invention, the catalyst (eg SnCl₄) may be charged to the reactor andtreated with a known volume of HF to fluorinate the catalyst prior tothe start of the reaction; for example prior to any continuous feed ofthe reagents for step (a). During this pre-fluorination step, any HClgenerated can be removed from the reactor.

The concentration of the catalyst can vary within wide limits. As anon-limiting example, the catalyst concentration may be from about 10 toabout 25 wt % in HF, such as from about 15 to about 20 wt % in HF

Step (a) is preferably conducted in a non-aqueous or anhydrousenvironment. It is therefore preferable to use anhydrous HF in step (a).

The ratio of HF to 250fb in step (a) can vary within wide limits. Theratio of HF to 250fb may, for example, be from about 1:1 to about 20:1,such as about 5:1 to about 15:1, for example about 10:1.

250fb is typically fed into the reactor for step (a) in the liquidphase.

Step (a) can be conducted as a batch reaction, a continuous reaction oras a semi-continuous reaction. Use of a semi-continuous reaction ispreferred because this allows for purging/cleaning and replacement ofthe catalyst as necessary.

An example of a possible arrangement of reactors of semi-continuousoperation of step (a) is to use 2 or more, for example, 3, 4 or 5reactors in parallel arranged so that each of the reactors may beindependently switched off and isolated to allow purging and/or cleaningwhile the other reactor(s) continue to operate.

For example, 3 reactors, such as 3 reactors each with a volume of fromabout 5 m³ to about 15 m³, for example, about 10 m³ may be used to carryout step (a). In this arrangement, the reaction rate may beapproximately 150 to 350 kg/m³h, such as approximately 200 to 300kg/m³h. It is expected that with this arrangement, it will be necessaryto purge/clean the reactor(s) every 3 to 10 days, for example aboutevery 4 or 5 days.

Any suitable reactor may be used for step (a). An example of a suitablereactor is a steady state, continuously stirred tank reactor.

In one advantageous arrangement, the reactor(s) used for step (a) may beconnected to a rectifying column(s). Lighter compounds such as unusedHF, 253fb and HCl generated during the reaction leave the reactor(s) viathe rectifying column(s), while heavier compounds remain in thereactor(s). In this arrangement, intermediates and/or by-products, suchas 251fb (CH₂ClCH₂CCl₂F) or 252fc (CClF₂CH₂CH₂Cl) may be condensed inthe rectifying column and thus be returned to the reactor, where theymay be fluorinated further.

In one particular arrangement, the HF fed may be introduced into thereactor via the top of the rectifying column. Without wishing to bebound by theory, it is believed that this can reduce fouling within thecolumn by lowering organics concentrations.

The product stream exiting the step (a) reactor, for example via therectifying column, can be subjected to one or more separation and/orpurification steps before being passed to the second reactor or beforebeing stored prior to passing to the second reactor. Alternatively, theproduct stream may be passed directly to the second reactor or may bestored without being subjected to separation and/or purification.

If the product stream from the step (a) reactor is subjected toseparation and/or purification any suitable separation and/orpurification techniques may be used. Suitable techniques include, butare not limited to distillation, phase separation, scrubbing andadsorption, eg using molecular sieves and/or activated carbon.

The product stream from the step (a) reactor may be subjected todistillation to separate HCl from the HF and organics. The HCl may thenbe recovered.

HF can be separated from a product stream comprising 253fb by, forexample, phase separation and/or use of an acid scrubber such as a H₂SO₄scrubber, for example a H₂SO₄ scrubber operated above ambienttemperature. HF separated in this way can be recycled back into the step(a) reactor. If an acid scrubber is used, the product stream comprising253fb may be passed through a molecular sieve or a sofnolime packed bedto remove traces of acid.

In an example of purification process that may be used in the presentinvention, the product stream from step (a) may be subjected todistillation to remove or reduce the concentration of HCl and then phaseseparation to separate the HF from the product stream comprising 253fb(the organics). The product stream may then optionally be passed througha H₂SO₄ scrubber.

The product stream may optionally be cooled before being subjected tophase separation or the phase separation may take place at or belowambient temperature.

The 253fb produced in step (a) may be condensed and stored, for examplein a buffer tank, prior to use in step (b).

It has been found by the inventors that an HF/253fb azeotrope ornear-azeotrope may be present in the product stream produced in step(a).

By azeotrope or azeotropic composition, we mean a binary compositionwhich at vapour-liquid equilibrium has the same composition in both theliquid and vapour phase, and whose boiling point is lower than that ofeither of the pure components. By near-azeotrope or near-azeotropiccomposition (e.g. a near-azeotropic composition of 253fb and HF), wemean a composition that behaves similarly to an azeotrope composition(i.e. the composition has constant boiling characteristics or a tendencynot to fractionate upon boiling), but may not have all of the propertiesof an azeotrope, for example binary liquid compositions whose vapourpressure is above that of the pure component with the lower boilingpoint (e.g. HF compared to 253fb) when measured at equivalenttemperature, but whose equilibrium vapour composition may differ fromthe liquid composition.

In essence, at a given pressure, a boiling azeotrope or near azeotropecomposition has the same constituent proportions in the vapour phase asin the boiling liquid phase. This means that no (or substantially no)fractionation of the components in the liquid composition takes place.

In the present invention, upon formation of the reaction product 253fbin step (a), HF may be present in an effective amount to form anazeotropic composition with 253fb. By effective amount, it is meant thatHF and 253fb are present in suitable ratios in order to form azeotropeor near azeotrope compositions.

A binary azeotrope composition has been identified between HF and R253fb(see FIGS. 1 to 3). Compositions comprising from about 65 mol % to about90 mol % HF and from about 35 mol % to about 10 mol % 253fb have beenshown to form azeotropes at temperatures of from about −25° C. to about+70° C., such as compositions comprising from about 70 mol % to about 85mol % of HF and from about 30 mol % to about 15 mol % of 253fb.

Additionally, near azeotrope compositions have been identified betweenHF and R253fb, wherein HF is present in an amount of from about 55 mol %to about 95 mol % and 253fb is present in an amount of from about 45 mol% to about 5 mol %. Such near-azeotrope compositions exist acrosstemperatures ranging from about −25° C. to about +70° C.

For example, it has been found that a composition consisting of about 75mol % HF and about 25 mol % 253fb is azeotropic at 70° C. and 600 kPa (6bara)

The present inventors have also found that phase separation, for examplephase separation at temperatures below approximately 30° C. can be usedto separate the HF and 253fb in the azeotropic or near-azeotropiccomposition. The HF phase separated may comprise 88 to 100 mol % HF,such as about 90 mol % HF. This separation step enables the HF to berecycled to step (a).

In one aspect of the invention, step (b), 253fb is dehydrochlorinated toproduce 1243zf in the vapour phase in the presence of a catalyst. Aprocess in which the product stream from reaction (a) is purified asdescribed above before 253fb is dehydrochlorinated to produce 1243zf inthe vapour phase in the presence of a catalyst is envisaged.

When step (b) is conducted in the vapour phase in the presence of acatalyst any suitable catalyst may be used. Suitable catalysts maycomprise activated carbon, alumina and/or chromia or zinc/chromia.Examples of suitable catalyst include activated carbon, Pt/carbon,Pd/carbon, Au/carbon, Pd/alumina, Ni/alumina, Pt/alumina, Cr/alumina orZn/chromia.

The inventors have found that the use of a catalyst in step (b) in thevapour phase enables the dehydrohalogenation step to be conducted usingless forcing conditions (e.g. lower temperature and/or pressure and/orresidence time) than would otherwise be necessary.

Catalysts suitable for use in step (b) in the vapour phase in thepresent invention can be obtained from commercial sources.

By the term “zinc/chromia catalyst” we mean any catalyst comprisingchromium or a compound of chromium and zinc or a compound of zinc. Suchcatalysts are known in the art, see for example EP-A-0502605,EP-A-0773061, EP-A-0957074 and WO 98/10862, which are incorporated byreference herein.

Typically, the chromium or compound of chromium present in thezinc/chromia catalysts of the invention is an oxide, oxyfluoride orfluoride of chromium such as chromium oxide.

The total amount of the zinc or a compound of zinc present in thezinc/chromia catalysts of the invention is typically from about 0.01% orabout 0.5% to about 25%, preferably 0.1% or about 1% to about 10%, fromabout 2 to 8% by weight of the catalyst, conveniently 0.01% to 6% zinc,for example about 4 to 6% by weight of the catalyst.

It is to be understood that the amount of zinc or a compound of zincquoted herein refers to the amount of elemental zinc, whether present aselemental zinc or as a compound of zinc.

The zinc/chromia catalysts used in the invention may include anadditional metal or compound thereof. Typically, the additional metal isa divalent or trivalent metal, preferably selected from nickel,magnesium, aluminium and mixtures thereof. Typically, the additionalmetal is present in an amount of from 0.01% by weight to about 25% byweight of the catalyst, preferably from about 0.01 to 10% by weight ofthe catalyst. Other embodiments may comprise at least about 0.5% byweight or at least about 1% weight of additional metal.

When step (b) is conducted in the vapour phase in the presence of acatalyst, this step is typically conducted at a temperature of fromabout 150° C. to about 450° C., such as from about 150° C. to about 400°C., e.g. from about 200° C. to about 350° C., or from about 150° C. toabout 250° C.

For example, the catalyst used in step (b) may comprise activated carbonand the process of step (b) may be conducted at a temperature from about250° C. to about 350° C., preferably 250° C. to about 300° C.

In the process of the invention, the catalyst used in step (b) may bepre-fluorinated, i.e. fluorinated prior to use. Any suitablepre-fluorination technique may be used, for example, the catalyst may bepre-fluorinated by passing HF over the catalyst prior to the catalystbeing contacted with 253fb.

For example, the catalyst used in step (b) may be a pre-fluorinatedZn/chromia catalyst, such as pre-fluorinated ZnO/Cr₂O₃, and the processof step (b) may be conducted at a temperature from about 250° C. toabout 350° C.

In the process of the invention, step (b) may also be conducted byco-feeding the HF and 253fb feeds. For example, the catalyst used instep (b) may be pre-fluorinated, such as a pre-fluorinated ZnO/Cr₂O₃catalyst, and HF and 253fb co-fed over the pre-fluorinated catalyst.

If the product stream from reaction (a) is purified as described abovebetween step (a) and (b), step (b) may alternatively be conducted in theliquid phase or in the vapour phase in the presence of an inert materialsuch as porcelain, quartz, alumina or Inconel mesh to aid heat transfer.Step (b) may, for example, be a thermal dehydrochlorination reactionconducted in the absence of a catalyst.

If step (b) is a thermal dehydrochlorination reaction this reaction istypically conducted at a temperature of from about 300 to about 800° C.,such as from about 400 to about 600° C., eg from about 450 to about 550°C.

When step (b) is conducted in the vapour phase, it is typicallyconducted at atmospheric, sub- or super atmospheric pressure, such as ata pressure of from about 0 kPa to about 3000 kPa (about 0 to 30 barg),preferably from about 100 kPa to about 200 kPa (about 1 to about 20barg).

The reaction time for the 1243zf preparation process (step (b)) isgenerally from about 1 second to about 100 hours, preferably from about10 seconds to about 50 hours, such as from about 1 minute to about 10 or20 hours in the vapour phase.

In a continuous process, typical contact times of the catalyst with thereagents is from about 1 to about 1000 seconds, such from about 1 toabout 500 seconds or about 1 to about 300 seconds or about 1 to about50, 100 or 200 seconds.

If step (b) is conducted in the liquid phase it is typically conductedin an aqueous environment in basic conditions. Any suitable base may beused to provide the basic environment. For example, aqueous NaOH may beused at a concentration of from about 10 to about 30 wt %. The liquidphase reaction typically takes place in the presence of a catalyst suchas a phase transfer catalyst. Suitable phase transfer catalysts includequaternary ammonium salts such as dodecyltrimethylammonium chloride.

The liquid phase reaction (b) is typically conducted at a temperature offrom about 30 to about 100° C., for example from about 50 to about 80°C. and a pressure of from about 100 kPa to about 300 kPa (about 1 toabout 3 barg)

The dehydrohalogenation step can be carried out in any suitableapparatus, such as a static mixer, a stirred tank reactor or a stirredvapour-liquid disengagement vessel. Preferably, the apparatus is madefrom one or more materials that are resistant to corrosion, e.g.Hastelloy®, Inconel® or a fluoropolymer lined vessel.

The dehydrohalogenation step may be carried out batch-wise or(semi-)continuously. Preferably, the step is carried out continuously.

The product stream exiting the step (b) reactor can be subjected to oneor more separation and/or purification steps.

If the product stream from the step (b) reactor is subjected toseparation and/or purification any suitable separation and/orpurification techniques may be used. Suitable techniques include, butare not limited to distillation, phase separation, scrubbing andadsorption, eg using molecular sieves and/or activated carbon.

In one purification method, the product stream from the step (b) reactoris subjected to one or more distillation steps. For example, the productstream from the step (b) reactor may be subjected to three distillationsteps. A first distillation step may be used to separate HCl forrecovery. A second distillation step may be used to remove lightby-products. A third distillation step may be used to separate outunreacted 253fb from the product; the unreacted 253fb can be recycledback into the step (b) reactor.

The 1243zf product can be stored for future use or can be passeddirectly into a further reactor.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 to 3 show the results obtained when measuring the vapourpressure of varying compositions of HF and 253fb over a temperaturerange of −25° C. to +70° C. The invention will now be illustrated withthe following non-limiting examples.

The invention will now be illustrated by the following non-limitingExamples.

EXAMPLE 1 Catalytic (Activated Carbon) Dehydrochlorination of 253fb

The carbon-based catalysts in Table 2 were ground to 0.5-1.4 mm and 2 mLcharged to an Inconel 625 reactor (0.5″ OD×32 cm) supported by Inconelmesh. The catalysts were pre-dried at 200° C. for at least 2 hours undera flow of N₂ (60 ml/min) at atmospheric pressure then the reactortemperature was increased to 250° C. and the nitrogen reduced to 30ml/min and diverted to the reactor exit. A flow of 253fb(3-chloro-1,1,1-trifluoropropane, 99.09%) was fed over the carboncatalysts by sparging liquid 253fb at 10° C. with 4-6 ml/min nitrogen,yielding 253fb vapour flows of 1-2.5 ml/min. After allowing the reactionto run for 30 min, reactor off-gases were sampled into deionised waterand analysed by GC, to give the conversion of 253fb and selectivity to1243zf results shown in Table 2. The experiment was also repeated at 300and 350° C. for selected catalysts.

TABLE 1 Results for 253fb dehydrochlorination to 1243zf with activatedcarbon catalysts 250° C. 300° C. 350° C. TR 253fb 1243zf 253fb 1243zf253fb 1243zf Catalyst Ref # Conversion % Selectivity % Conversion %Selectivity % Conversion % Selectivity % Act. Carbon 2367 9.82 100.0037.23 100.00 Act. Carbon 2091 17.12 100.00 64.24 100.00 Act. Carbon 203226.10 100.00 73.01 100.00 Act. Carbon 1968 55.37 100.00 96.87 100.00Act. Carbon 2019 20.03 100.00 66.12 100.00 Act. Carbon 2366 64.60 100.0093.25 100.00 96.99 99.55 1.5% 2630 14.06 100.00 17.87 97.79 36.62 33.77Pd/Carbon 0.8% 2629 48.53 100.00 93.08 100.00 88.69 97.23 Pd/Carbon 0.3%2634 47.09 100.00 94.60 99.40 Au/Carbon

EXAMPLE 2 Catalytic Dehydrochlorination of 253fb Investigating theEffect of Pre-Fluorination, with/without HF Co-Feed on ZnO/Cr₂O₃

A ZnO/Cr₂O₃ catalyst was ground to 0.5-1.4 mm and 2 mL charged to anInconel 625 reactor (0.5″ OD×32 cm) supported by Inconel mesh. Thecatalyst was pre-dried at 200° C. for at least 2 hours under a flow ofN₂ (60 ml/min) at atmospheric pressure. Three experiments carried out induplicate were as follows:

Without Pre-Fluorination:

The nitrogen flow was reduced to 30 ml/min and diverted to the reactorexit and the reactor temperature increased to 250° C. A flow of 253fb(3-chloro-1,1,1-trifluoropropane, 99.09%) was fed over the catalyst bysparging liquid 253fb at 10° C. with 10-12 ml/min nitrogen, yielding253fb vapour flows of 4-5 ml/min. After allowing the reaction to run for30 min, reactor off-gases were sampled into deionised water and analysedby GC, to give the conversion of 253fb and selectivity to 1243zf resultsshown in Table 2. The experiment was also repeated at 300 and 350° C.

Pre-Fluorinated:

HF at 30 ml/min was passed over the catalyst along with 60 ml/minnitrogen at 300° C. for one hour. The nitrogen flow was then directed tothe reactor exit leaving neat HF passing over the catalyst. Thetemperature was slowly ramped to 360° C. and held for 10 hours. Afterthis time the temperature was reduced to 300° C. and the flow of HFstopped and replaced with 30 ml/min nitrogen, for 1 h. The flow ofnitrogen was then diverted to the reactor exit then a flow of 253fb(3-chloro-1,1,1-trifluoropropane, 99.09%) was fed over the catalyst bysparging liquid 253fb at 10° C. with 10-12 ml/min nitrogen, yielding253fb vapour flows of 4-5 ml/min. After allowing the reaction to run for30 min, reactor off-gases were sampled into deionised water and analysedby GC, to give the conversion of 253fb and selectivity to 1243zf resultsshown in Table 2.

Pre-Fluorinated and HF Co-Feed:

Pre-fluorination as described above. After this time the temperature wasreduced to 250° C. and the flow of HF maintained over the catalyst. Aflow of 253fb (3-chloro-1,1,1-trifluoropropane, 99.09%) was fed over thecatalyst by sparging liquid 253fb at 10° C. with 10-12 ml/min nitrogen,yielding 253fb vapour flows of 4-5 ml/min. After allowing the reactionto run for 30 min, reactor off-gases were sampled into deionised waterand analysed by GC, to give the conversion of 253fb and selectivity to1243zf results shown in Table 3. The experiment was also repeated at 300and 350° C.

TABLE 3 Results for 253fb dehydrochlorination to 1243zf with ZnO/Cr₂O₃catalyst HF 253fb 250° C. 300° C. 350° C. Pre- flow flow 253fb 1243zf253fb 1243zf 253fb 1243zf fluorinated ml/min ml/min Conversion %Selectivity % Conversion % Selectivity % Conversion % Selectivity % Yes31 5.3 41.20 85.15 93.50 99.47 93.57 99.29 Yes 31 5.1 40.19 85.00 87.9699.01 98.02 99.79 No 0 4.1 6.93 93.30 8.74 90.32 5.53 77.57 No 0 4.71.94 83.70 6.01 86.08 7.34 82.15 Yes 0 5.1 22.00 98.03 Yes 0 4.9 19.5797.57

Overall there was an improvement in the conversion and slightly higherselectivity to 1243zf when the catalyst was pre-fluorinated and 253fbco-fed with HF.

EXAMPLE 3 Azeotrope Identification

A binary azeotrope between HF and 253fb was identified by a study of thevapour-liquid equilibrium of binary mixtures over a temperature range of−25° C. to +70° C. using a constant volume apparatus.

The experimental data were measured in a static constant volumeapparatus consisting of a vessel of precisely known internal volume(32.57 ml) located in a temperature-controlled metal block. A magneticstirring device was located inside the vessel. Refrigerated fluid waspassed through the block to allow precise control of temperature insidethe vessel. The cell was evacuated then known amounts of compositions ofHF and 253fb were charged to the cell. The cell was then varied stepwisefrom about −25° C. to +70° C. At each step the cell temperatures andpressure were logged and recorded when stable conditions were reached.

The compositions studied are given in Table 3 below. The phase behaviourof these compositions at three exemplary temperatures, being −25° C.,+30° C. and +70° C. is illustrated in FIGS. 1 to 3. The graphs in FIGS.1 to 3 show that a constant vapour pressure is reached at compositionswherein HF is present in an amount of from about 55 mol % to about 95mol % and 253fb is present in an amount of from about 45 mol % to about5 mol %, which is consistent with what would be expected of azeotropiccompositions. This trend is evidenced across all temperature rangestested.

TABLE 3 Mole fraction Mole fraction % w/w R253fb HF HF 1.000 0.000 0.0000.971 0.029 0.452 0.952 0.048 0.752 0.906 0.094 1.546 0.855 0.145 2.4880.766 0.234 4.417 0.708 0.292 5.867 0.638 0.362 7.902 0.558 0.442 10.6960.558 0.442 10.696 0.434 0.566 16.442 0.335 0.665 23.058 0.253 0.74730.790 0.150 0.850 46.130 0.126 0.874 51.102 0.100 0.900 57.743 0.0730.927 65.671 0.052 0.948 73.330 0.016 0.984 90.390 0.000 1.000 100.000

EXAMPLE 4

A feed composition of HF and 253fb was charged to a whitey bomb,agitated, and placed in a chilled bath at constant temperature. Thesystem was left overnight to achieve thermal and phase equilibrium.Consecutive samples were withdrawn from the base of the whitey bomb,slowly, every half an hour over a total period of 4 hours so as not todisturb the phase equilibrium in the bomb, and analysed to determine HFconcentration. The results shown in Table 5 demonstrate the separationof HF and 253fb into two liquid phases.

Initial charge 99.7 g Feed composition 81.91 mol % HF Temp −25° C.

TABLE 4 Sample Sample mass (g) Mol Frac HF 1 8.2 33.91* 2 7.2 16.64 325.9 17.53 4 8.2 15.06 5 12.3 96.11 6 8.5 97.53 7 7.8 97.47 8 10.4 97.019 Residual mass Not analysed *Note, the geometry of the offtake line atthe base of the bomb means that the initial contents of the offtake linedo not reach phase equilibrium with the bulk contents within the bomb.This results in an initial sample which contains high levels of HF.

1. A process for preparing 3,3,3-trifluoropropene (1243zf ), the processcomprising: (a) fluorinating CCl₃CH₂CH₂Cl (250fb) to produce a reactionproduct comprising CF₃CH₂CH₂Cl (253fb) in the liquid phase in a firstreactor, using HF as the fluorinating agent; and (b) dehydrohalogenadrig253fb to produce 1243zf in the vapour phase in the presence of acatalyst in a second reactor.
 2. A process according to claim 1, whereinthe catalyst used in step (b) comprises activated carbon, alumina and/orchromia or zinc/chromia.
 3. A process according to claim 2, wherein thecatalyst used in step (b) is activated carbon, Pd/carbon, Pt/carbon,Au/carbon, Pd/alumina, Ni/alumina, Pt/alumina, Cr/alumina or Zn/chromia.4. A process according to claim 1, wherein the catalyst used in step (b)is pre-fluorinated.
 5. A process according to claim 1, wherein step (b)is conducted at a temperature from about 250° C. to about 350° C.
 6. Aprocess according to claim 5, wherein step (b) is conducted at atemperature of from about 250° C. to about 300° C.
 7. A processaccording to claim 2, wherein the catalyst used in step (b) comprisesactivated carbon and the process of step (b) is conducted at atemperature from about 250° C. to about 300° C.
 8. A process accordingto claim 4, wherein the catalyst used in step (b) is a pre-fluorinatedZn/chromia catalyst and the process of step (b) is conducted at atemperature from about 250° C. to about 350° C.
 9. A process accordingto claim 1, wherein in step (b) HF is co-fed with 253fb.
 10. A processaccording to claim 1, wherein the reaction product comprising 253fbproduced in step (a) is subjected to one or more purification stepsbefore step (b).
 11. A process for preparing 3,3,3-trifluoropropene(1243zf), the process comprising: (a) fluorinating CCl₃CH₂CH₂Cl (250fb)to produce a reaction product comprising CF₃CH₂CH₂Cl (253fb) in theliquid phase in a first reactor, using HF as the fluorinating agent; and(b) dehydrohalogenating 253fb to produce 1243zf in a second reactor;wherein the reaction product comprising 253fb produced in step (a) issubjected to one or more purification steps before step (b).
 12. Aprocess according to claim 9, wherein the purification steps comprisesubjecting product stream from step (a) to distillation to remove orreduce the concentration of HCl and then phase separation to separate HFfrom the product stream comprising 253fb and optionally passing thestream comprising 253fb through a H₂SO₄ scrubber.
 13. A processaccording claim 11, wherein HF is recycled to step (a).
 14. A processaccording to claim 1, wherein the first reactor is connected to arectifying column.
 15. A process according to claim 13, wherein at leastpart of the HF is introduced into the first reactor via the top of therectifying column.
 16. A process according to claim 11, wherein unusedHF is separated from the reaction product produced in step (a) andrecycled to the first reactor.
 17. A process according to claim 1,wherein step (a) is conducted at a temperature of from about 40 to about70° C.
 18. A process according to claim 1, wherein step (a) is conductedat a pressure of from about 200 kPa to about 700 kPa.
 19. A processaccording to claim 1, wherein step (a) is conducted in the presence of apolymerisation inhibitor and/or retarder.
 20. A process according toclaim 19, wherein the polymerisation inhibitor and/or retarder isselected from the group consisting of: cyclic ketone or quinone-basedaromatic compounds, nitro- or nitrogen-containing compounds, andsulphur-containing compounds.
 21. A process according to claim 20,wherein the cyclic ketone is cyclobutanone or cyclohexanone or a mixturethereof.
 22. A process according to claim 1, wherein step (a) isconducted in the presence of a Lewis acid catalyst.
 23. A processaccording to claim 22, wherein the catalyst is SnCl₄.
 24. A processaccording to claim 1, wherein step (a) is a semi-continuous reaction.25. A process according to claim 1, wherein the process in step (b) iscontinuous.
 26. An azeotropic or near-azeotropic composition comprisingHF and 253fb.
 27. An azeotropic or near-azeotropic compositionconsisting of HF and 253fb.
 28. An azeotropic or near-azeotropiccomposition according to claim 27 consisting of from about 70 mol % toabout 85 mol % HF and from 30 mol % to 15 mol % 253fb.
 29. A compositionthat is azeotropic at 70° C. and 600 kPa, which consisting of about 75mol % HF and about 25 mol % 253fb.
 30. (canceled)